1. Field of the Invention
The present invention relates to a process for producing magnetic resonance images using actual sample trajectories to produce images without errors caused by sampling errors in the spatial frequency space (the k-space). The invention permits the measuring of magnetic resonance imaging sample trajectories in situ from the imaged subject or patient directly and efficiently. The process measures the k-space trajectory using phase values of acquired magnetic resonance signals.
2. Description of the Prior Art
The process of producing images based on readings from magnetic resonance equipment and a number of k-space calibration approaches have been described in both patents and printed literature (1-5). One prior method calibrates the test gradient waveforms using self-encoding gradients. This prior method measures k-space trajectory by analyzing the temporal locations of echo peaks produced by the combination of self-encoding gradients and a test gradient waveform.
Another prior approach measures the actual k-space trajectories by recording signals from a small phantom at a number of locations inside the imaging field (5-8). The time varying magnetic field at the probe location is recorded via phase differences of the acquired data and k-space trajectories are then obtained by solving a set of linear equations.
Still another prior art technique for measuring the actual sample trajectories involves measuring the driving currents of gradient amplifiers (9, 10). Other techniques currently being researched include an approach which establishes landmarks in the k-space by applying short radio frequency pulses at selected times during the k-space traversal (11). Actual k-space trajectories are then determined from the established landmarks.
There have also been many patents in the field of magnetic resonance imaging utilizing information derived from k-space. U.S. Pat. No. 5,621,321 entitled "Magnetic Resonance Scan Calibration Method for Ultra Fast Image Acquisition" describes a method in which data lines are one-dimensionally transformed into a frequency encoded direction. Other techniques have been used for producing images from magnetic resonance techniques such as the technique using gradient echoes and spin echoes or (Grase). This technique, however, selectively phase encodes and time shifts the echo responses and occurrence so as to smoothly distribute unwanted phase shift from field inhomogeneity and/or chemical phase shift effects over the phase encoded dimension in k-space. U.S. Pat. Nos. 5,680,045 and 5,270,654 utilized this Grase technique.
Each of the prior mentioned approaches has its shortcomings. The self-encoding approach takes a long time, and its accuracy is dependent on the accuracy of realized self-encoding gradient amplitude. Gradient performance non-linearity is not accounted for. The RF landmark method is also time consuming and because of the limits in the temporal resolution of the RF impulses and limits on the RF energy deposition, only a few k-space locations are marked during each acquisition. Large numbers of repetitions are necessary to measure an entire k-space trajectory. The small phantom method requires separate phantom experiments and the gradient current monitoring does not account for eddy currents.
Magnetic resonance images are reconstructed from discrete samples of the imaged object's distribution in the spatial frequency domain (the k-space). Actual sampling trajectory in the k-space often deviates from that specified by the ideal spatial encoding gradient waveforms due to imperfections in gradient amplifier performance, readout timing errors, and eddy currents induced by gradient pulses. The resulting misregistrations in k-space sample locations cause image artifacts and distortions in images reconstructed using ideal k-space sample locations. Echo planar imaging, fast spiral scan MRI, and other fast non-Cartesian scan MRI techniques that are widely used in many important imaging applications such as functional brain imaging, interventional imaging, etc., are especially susceptible to this type of image errors.
Modern magnetic resonance scanners use actively shielded gradients and electronic compensation measures to reduce and compensate for induced eddy currents. Despite these corrections, substantial k-space sample trajectory deviations still exist, especially for fast imaging applications that require fast gradient switching and/or ramping (e.g., echo planar imaging) or, use complicated non-Cartesian sampling patterns (e.g., spiral scanning magnetic resonance imaging). In the former case, gradient amplifiers may be driven into a nonlinear operating range causing gradient waveform distortions and thereby k-space sample trajectory deviations. In the latter case, the use of complicated non-Cartesian scan patterns makes eddy current compensation by common measures, mostly adjusted for proper compensation of trapezoidal waveforms, less effective. In most of these cases, due to higher than normal sampling rates, image errors caused by readout timing errors are amplified.
Effects of k-space sample location misregistration can be corrected by reconstructing images using the actual k-space trajectories by regridding interpolation and Fast Fourier Transforms (FFT) (12). Because of aforementioned shortcomings of prior art methods for the measurement of actual k-space trajectories, such measurements are only made for a limited number of scan orientations and field of views. Imaging in other orientations or with other field of views suffers from significant errors due to k-space trajectory deviations. Thus, there exists a need for a technique that can measure k-space trajectories in situ to allow imaging in arbitrary orientations that best fit the requirements of each particular study.